In prior art, sheet processing apparatuses which checks various sheets such as paper money have been put to practical use. The sheet processing apparatuses have an image reading apparatus which reads images of sheets. The sheet processing apparatuses takes in sheets which are put into an inlet one by one, and carries the sheets to the image reading apparatus.
The image reading apparatus includes an illumination module (illumination apparatus) and a line image sensor. The image reading apparatus irradiates each sheet which is carried in a predetermined direction with light by the illumination apparatus. The image reading apparatus reads transmission light or reflection light of the light applied to the carried sheet by the line image sensor, and obtains an image of the sheet.
The illumination apparatus irradiates a surface of each sheet as a test subject with light. In this case, the illumination apparatus desirably has an illumination distribution which is uniform in the longitudinal direction of the line image sensor. When high-speed and accurate processing is performed, the illumination module is required to have high and uniform illumination to reduce the load of image processing and signal processing.
The illumination module has a light source such as a halogen light source and a fluorescent light, and a light guide (light-guiding member) using light fiber. In addition, in recent years, illumination modules which have a light source using light emitting diodes (hereinafter simply referred to as LEDs) are having been put to practical use. The illumination modules have LEDs which are arranged in a straight line or rows and columns.
However, in a light source device in which light-emitting devices such as LEDs are arranged, the illumination distribution on a test subject is a distribution obtained by superposing illumination distributions of respective light-emitting devices in a three-dimensional manner. Therefore, in the case of using light-emitting devices which have a high-directional light distribution and have a structure of uniting an LED with a lens, the illumination distribution of each light-emitting device has a narrow range and a bell shape, and the illumination distribution obtained by superposing the illumination distributions of the light-emitting devices have a wave shape, and has a problem of generating non-uniform illumination. In addition, since the light-emitting devices have high directivity, unevenness in the total illumination distribution is easily caused by difference in position and angle between the individual light-emitting devices.
On the other hand, in the case of using light-emitting devices which have low directivity and have a structure in which an LED is not united with a lens, an area to which one light-emitting device applies light is increased. Therefore, although a part around the center of the test subject is irradiated with light beams which come from both directions as viewed from the center of the test subject, a part around the edge of the test subject is irradiated with only light which comes from one direction in which the LED exists as viewed from the edge. This causes the problem of unevenness in illumination, that is, the part around the center of the test subject has high illumination distribution, and the illumination gradually decreases toward the edge.
As a method of preventing unevenness in illumination, there is a method of reducing a light-emitting quantity of light-emitting devices around the center of the test subject by flowing a small quantity of current through them, increasing a light-emitting quantity of light-emitting devices around the edge by flowing a large quantity of current through them, and thereby balancing the illumination. However, this method cannot sufficiently exhibit the brightness performance of the light-emitting devices.
Although there is also a method of increasing the light-emitting quantity around the edge by increasing the density of the light-emitting devices around the edge, and thereby preventing decrease in illumination, it is physically impossible to increase the density of the light-emitting devices more than the size of each light-emitting device, and thus increase in the light-emitting quantity is limited.
In addition, there is an illumination apparatus including a reflection member which has an oval-arc-shaped cross section in the sheet carrying direction, to collect light emitted from the light source into a reading range of the line image sensor. However, in the case of using a reflection member having an oval-arc-shaped cross section, the illumination distribution has a pointed peak.
When the illumination distribution has a pointed peak, the range in which the illumination apparatus has uniform illumination is narrowed. Therefore, there is the problem that it is difficult to install the line image sensor and the illumination apparatus such that the reading range of the line image sensor agrees with the peak of the illumination distribution.
In addition, since the peak of the illumination distribution is pointed, the illumination distribution is not uniform in the reading range of the line image sensor. This causes the problem that uneven brightness may occur in the read image according to the sheet carrying state or the like.